The present invention relates generally to operations performed in subterranean wells and, in an embodiment described herein, more particularly provides apparatus and methods for underbalanced drilling and completion of wells.
There are several recognized advantages to drilling and completing a well in an underbalanced condition, that is, in a condition in which fluid pressure in a wellbore is less than fluid pressure in a formation intersected by the wellbore. For example, the underbalanced condition prevents fluid loss from the wellbore into the formation and prevents some types of damage to the formation which may be caused by infiltration of the wellbore fluid into the formation. An overview of underbalanced completion practices and their advantages may be found in an article entitled "Underbalanced Completions Improve Well Safety and Productivity" by Tim Walker and Mark Hopmann (World Oil, November, 1995), which is incorporated herein by this reference.
Unfortunately, apparatus and methods which facilitate convenient, economical and safe underbalanced well operations are not presently widely available. For example, currently available apparatus designed to permit safe tripping in and out of drill strings and production tubing strings rely either on complex, expensive and unreliable mechanisms or on adapted surface-controlled devices, such as subsurface safety valves, which must be installed relatively near the surface or face a significant risk of damage to control lines attached thereto if installed relatively deep in the well. Thus, a need exists for apparatus which will safely and conveniently facilitate underbalanced well operations.
In particular, a need exists for a well control valve which is operable upon passage of a tool therethrough. The tool may be attached to a drill string, production tubing string, or other conveyance. In this manner, the valve may isolate a formation intersected by a wellbore in an underbalanced condition from the remainder of the wellbore while the tubular string is tripped in or out of the wellbore. The valve should be capable of being installed near the formation, without compromising its operability or reliability.
Where the valve is operated by applying a biasing force to the valve via a tubular string, and the tubular string includes a packer, the packer should be prevented from prematurely setting in the wellbore due to application of the biasing force. Therefore, it would be highly desirable to provide a packer setting tool which prevents premature setting of the packer, while also facilitating use of the packer in underbalanced well operations.